Vehicles are commonly detected by their influence on the inductance of an a.c. energised detector loop laid in the roadway or other place where detection is required. In some circumstances it is necessary to continue to signal the arrival of a vehicle at the detector loop, assuming the vehicle remains stationary, for at least a selected period of time--the presence time. It may also be desired in some cases to have a "permanent" or indefinite presence time so that the presence indication is only cancelled upon the departure of the vehicle from the detector loop.
Vehicle detection is commonly done by making the detector loop the frequency determining element of an oscillator, and monitoring frequency (or oscillator period) changes to determine whether a vehicle is present in the vicinity of the loop. In performing such monitoring due allowance needs to be made for ambient changes affecting the oscillator frequency, which changes may be referred to as environmental drift.
The frequency changes may be monitored by regularly sampling the oscillator frequency and examining successive samples for a change that is taken to indicate the arrival of a vehicle. Environmental drift is allowed for on the general basis that changes due to it are at a slow rate relative to the rate of change due to the arrival of a vehicle. However, once the vehicle arrives it is conventional practice to hold as a reference a sample value obtained prior to arrival. Further samples are now compared with the reference to determine whether the vehicle has left the loop vicinity, this procedure occurring over the presence period after which the apparatus is restored to its normal sampling and examination routine.
Sampling may be performed by various digital techniques such as described in British Pat. Nos. 1,495,759, 1,448,967, 1,366,875. Analog techniques were previously known, for example, as disclosed in British Pat. No. 1,338,062. One concern of the present invention is with the provision of a sampling method and apparatus, particularly for use in the detection of the arrival of a vehicle.
During the presence period following vehicle detection, if the reference sample is not compensated for environmental drift and such drift occurs in the "detect" direction, i.e. influences the oscillator frequency in the same sense as the vehicle arrival, it may be that the sample obtained on vehicle departure is still indicating, falsely, a vehicle presence. On the other hand if the drift occurs in the "non-detect" direction, it may be that over an extended presence period the samples eventually obtained are taken as indicating, falsely, the departure of the vehicle.
Two previous solutions to these difficulties have been to either keep the presence time short, e.g. a few minutes, before restoring the detector loop to normal operation, or increment the reference value steadily in the detect direction in which case eventually the conditions required for a presence signal no longer apply. The first approach sets out to forget the vehicle presence after a short period; the second has the same result. A better solution based on digital sampling is described in British Pat. No. 1,398,937.
It will be shown hereinafter how the present invention may be put into practice to achieve "permanent" presence time while at the same time allowing for environmental drift during the presence period. As already mentioned, the invention is also concerned with detection procedures for the arrival of a vehicle and provides a method and apparatus to that end that is conveniently and compatibly used with the procedures applied during the presence period.